The Latest Developments on Powering Our World with Grass, Twigs, and Other Green Waste

April 9, 2020 |

By Michael Timko, Associate Professor of Chemical Engineering, Worcester Polytechnic Institute

Special to The Digest

The sound of scraping your plate of un-eaten food is a familiar one to all of us. For most people in the U.S., this food winds up in the local landfill. In some locations, composting options are available. But, the food left on our plates is energy dense, rich in starches, lipids, and other components, all begging to be made into biofuels. After years of development, bioenergy costs remain too high to compete with petroleum on price alone. Can converting food and other wastes to biofuel be a transition technology to ease our dependence on fossil fuels and into a bioeconomy?

Shifting Perspective on Waste to Biofuel

Even though much of what we consume creates waste, waste generation is not inevitable. Natural ecosystems treat waste as a resource, not a liability; for instance, a farmer might consider an apple that fell to the ground from a tree as waste, but the surrounding microbes, animals, and plants living around that apple consume it for nutrients and energy. Consequently, we can take a page out of nature’s book on how to perceive waste. A shift in perspective is needed to grasp this, and a key place to start is in waste research.

In my lab at Worcester Polytechnic Institute, my team, which includes collaborators from MIT, University of California, Riverside, Massachusetts-based Woods Hole Oceanographic Institution, and Florida-based Mainstream Engineering Corp, is broadening attempts to convert waste into environmentally friendly biofuels. In doing so, we hope to lower reliance on fossil fuels, cut the amount of municipal waste going into landfills, and reduce water pollution and unhealthy emissions from petroleum products and landfills.

Most recently, we received a $1,995,199 three-year grant from the Department of Energy and $275,000 from the Massachusetts Clean Energy Center to develop ways to significantly improve the yield of biofuel that can be created from food waste and municipal green waste, such as yard trimmings, leaves, and sticks. By combining the two kinds of waste, we are aiming to reach economies of scale that would be impossible to achieve with either stream on its own.

Anaerobic digestion vs. hydrothermal liquefaction

The main goal of our project is to develop a catalytic method for converting the food waste and yard waste components of municipal solid waste into an energy-dense oil. Anaerobic digestion has been used for decades to convert wet wastes into methane-rich biogas. However, anaerobic digestion cannot easily handle yard waste, especially if it contains whole biomass, such as wood. Moreover, anaerobic digestion is a slow process that requires days or weeks to complete and yields a product that must compete with abundant supplies of natural gas.

Instead, we are investigating the use of hydrothermal liquefaction, a process that uses moderate heat and high pressure to convert wet biomass into crude-like oil. This process is faster than anaerobic digestion and can handle feeds that are not suitable for anaerobic digestion. Like anaerobic digestion, hydrothermal liquefaction is compatible with wet feeds, meaning that wet food waste and yard waste feeds do not need to be dried prior to conversion. For feeds that contain greater than 50 percent water, eliminating the drying step is crucial to achieving a net return on energy input.

Improvement and catalysts

The hydrothermal liquefaction process converts carbon contained in the feed into products that separate into oil, gas, char, and water phases. We are working to improve the hydrothermal process to get the highest-quality oil possible, to divert the carbon lost to the water into the production of oil, and to minimize the amount of energy put into the process, while maximizing the energy produced.

To achieve this, we are adding various catalysts, such as red mud – an inexpensive abundant waste material created during the production of aluminum – and other inexpensive oxides with acid and base properties. The right catalyst seems to promote breakdown of polymers while promoting carbon-carbon coupling reactions that lead to molecular weight growth that converts would-be water-soluble products into oil soluble ones.

We hope to have demonstrated our process at pilot scale by the end of the three-year grant. The data and experience we gain from the pilot plant tests will better position our technology for follow-on investments from the corporate sector, government sources, and others.

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Category: Thought Leadership

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